DE102014115638B4 - Gas sensor element, gas sensor, and method for producing a gas sensor element - Google Patents

Abstract

A gas sensor element (7, 207) comprising:a substantially rectangular parallelepiped element body (70, 270) extending in a longitudinal direction and having in or at its front end region a detection portion (90) for detecting a specific gas contained in a gas to be measured; anda porous protective layer (17) provided on a front end surface (127) and side surfaces (21, 23, 111, 113) of the element body (70, 270) in such a manner that it covers at least the detection portion (90),wherein the gas sensor element (7, 207) has at least one gap (18) formed to separate the element body (70, 270) and the protective layer (17), the gap (18) being provided only in a front region (E1) located in front of a location (19) on the side surfaces of the element body (70, 270) where the protective layer (17) has its greatest thickness;wherein the gap (18) is formed at at least one corner point of four corner points (74) of a front end of the element body (70, 270) in such a manner that the gap extends over three surfaces of the element body (70, 270) defining the corner point; andwherein the protective layer (17) is in contact with at least a portion of the front end surface (127), and is in contact with at least a portion of the four side surfaces (21, 23, 111, 113) in the front region.

Description

Technical area

The present invention relates to a gas sensor element for detecting a specific gas contained in a gas to be measured, a gas sensor having the gas sensor element, and a method for manufacturing the gas sensor element.

General state of the art

A conventionally known gas sensor having a gas sensor element for detecting a specific gas contained in a gas to be measured is, for example, an oxygen sensor which is installed in an exhaust gas flow path such as an exhaust pipe of an internal combustion engine and is used in controlling combustion of the internal combustion engine by detecting an oxygen concentration in the exhaust gas. The oxygen sensor has, for example, a tubular metal case and a plate-like gas sensor element held in the metal case.

The gas sensor element includes a plate-like element body extending in the longitudinal direction and a protective layer formed of a porous material on the surface of the element body. The element body includes a detection portion provided in its longitudinally front portion and capable of detecting a specific gas contained in a gas to be measured. The protective layer is provided in a longitudinally front end portion of the element body in such a manner as to cover at least the detection portion.

The protective layer is provided to protect the element body. Direct adhesion of condensed water may break the heated element body by thermal shock. Therefore, by providing the protective layer to suppress direct adhesion of condensed water or the like to the element body, breakage of the element body can be suppressed. The protective layer has a certain thickness to allow condensed water to evaporate before it reaches the element body.

A known method for forming the protective layer of the gas sensor element is an immersion process in which a front end portion of the element body is immersed in a slurry prepared by mixing ceramic powder, water, and a pore-forming agent (for example, carbon powder); see Patent Document 1. The slurry applied to the element body is subjected to heat treatment and becomes the protective layer.

State of the art documents

Patent Document 1: Patent Disclosure Document (kokai) JP 2003 - 322632 A1 . JP2012-168030A1 discloses a gas sensor element and a gas sensor. US 2012 / 0 297 861 A1 discloses a gas sensor element and its manufacturing method, as well as a gas sensor using the gas sensor element.

Brief description of the invention

However, the above dipping process involves the following problem. Since the thickness of the protective layer decreases at certain locations, especially near four corner points at the front end of the element body, dipping must be performed several times to ensure sufficient thickness of the protective layer at these certain locations.

The repeated immersion imparts excessive thickness to those sections of the protective layer that could have a sufficient thickness of the protective layer with a single (single) immersion. Therefore, the total volume of the protective layer increases and the heat capacity of the protective layer increases accordingly, which, together with an increase in the time required for the activation of the sensor element, leads to unnecessary power consumption by a heating element.

One conceivable measure to solve this problem is to use a spraying process to spray the slurry after application of the slurry by a single immersion using a atomizer to those specific areas where a single dip does not provide sufficient thickness.

However, the spraying process tends to cause waste of the slurry; in particular, since the slurry is sprayed in the form of a mist, part of the slurry drops off without being applied to the element body. In addition, the spraying process involves the following problem: since the spray amount per unit time is small and therefore the working time for spraying becomes long, the steps for forming the protective layer become troublesome.

In view of the above problem, an object of the present invention is to provide a gas sensor element whose protective layer has a lower heat capacity than a conventional protective layer formed by an immersion process, and a gas sensor comprising the gas sensor element, as well as a method for manufacturing the gas sensor element.

The present invention, according to an embodiment, provides a gas sensor element comprising a rectangular parallelepiped element body and a porous protective layer. The element body extends in a longitudinal direction and has in or at its front end portion a detection portion for detecting a specific gas (for example, a gas component) contained in a gas to be measured (for example, a gas mixture). The porous protective layer is provided or arranged on or at a front end surface and side surfaces of the element body in such a manner as to cover at least the detection portion.

The gas sensor element has at least one gap formed to separate the element body and the protective layer (for example, the at least one gap is defined between the element body and the protective layer), the gap being provided only in a front region located in front of the location or place on the side surfaces of the element body where the protective layer has its greatest thickness.

The gap is provided at (for example, arranged adjacent to) at least one corner point of four corner points of a front end of the element body such that the gap extends over three surfaces of the element body defining the corner point, and the protective layer is in contact with at least a portion of the front end surface and is in contact with at least a portion of the four side surfaces in the front region.

In the gas sensor element thus constructed, the gap for separating the element body and the protective layer is provided at the corner point of the front end of the element body where the thickness of the protective layer is likely to become small so that the gap extends over three surfaces of the element body defining the corner point. Thereby, even if the thickness of the protective layer is small near the corner point, it is possible to suppress breakage of the corner point of the front end of the element body, which might otherwise result from a temperature shock resulting from adhesion of water. This is because the gap provided at the corner point causes condensed water adhering to the protective layer to penetrate into the protective layer bypassing the gap, thereby preventing the condensed water from reaching the corner point.

That is, compared with a conventional protective layer formed by an immersion process, the thickness, and therefore the heat capacity, of the protective layer can be reduced.

In addition, the gap is formed only in the front region located in front of the point or location on the side surfaces of the element body where the protective layer has its greatest thickness. Thanks to this feature, it is possible to prevent condensed water from reaching the corner point through the gap, and it is also possible to prevent deterioration of the adhesion between the protective layer and the element body in a region located behind the point on the side surfaces of the element body where the protective layer has its greatest thickness.

In addition, the protective layer is in contact with at least a portion of the front end face of the element body, and is in contact with at least a portion of the four side faces in the front area. Thereby, despite the fact that the gap formed in such a manner as to separate the element body and the protective layer is provided in the front area, deterioration of the adhesion between the protective layer and the element body itself in the front region can be suppressed.

Thus, the present invention can provide a gas sensor element having a protective layer whose heat capacity is lower than that of a conventional protective layer formed by an immersion process.

The "substantially rectangular parallelepiped element body" referred to in the present invention includes not only a rectangular parallelepiped element body but also an element body in the shape of a rectangular parallelepiped whose edges are chamfered. In the case of an element body with chamfered edges, "four vertices of a front end of the element body" mean ridge lines defined by the chamfers and the front end surface.

There is no particular limitation on the provision of the gap as long as the gap is provided at at least one of four corner points at the front end of the element body. The gap may be provided at a single corner point, or the gaps may be provided at the four corner points. Moreover, the gaps may be provided in a one-to-one relationship at the corner points, or a single gap may be provided across multiple corner points.

It is sufficient that the protective layer is in contact with at least a portion of the front end surface of the element body. Therefore, the protective layer may be in contact with a portion of the front end surface or may be in contact with the entire front end surface except for the gap.

It is sufficient if the protective layer is in contact with at least a portion of the four side surfaces in the front region. Therefore, the protective layer may be in contact with one side surface in the front region or may be in contact with each of the four side surfaces in the front region.

In the sensor element described above, the protective layer may be in contact with at least a portion of each of the four side surfaces in the front region.

When the protective layer is in contact with at least a portion of each of the four side surfaces in the front portion, it is possible to further suppress deterioration of the adhesion between the protective layer and the side surfaces of the element body in the front portion. This makes it even less likely that the protective layer will separate from the element body.

In the gas sensor element described above, the element body may be in contact with the protective layer through a portion of the front end surface and portions of the side surfaces in the front end region, wherein the total surface area of the portion of the front end surface and the portions of the side surfaces is half of the total surface area of the front end surface and the side surfaces.

In this case, a sufficiently large contact area between the element body and the protective layer can be ensured in the front end region. Therefore, it becomes even less likely that the protective layer will separate from the element body.

In the above-mentioned gas sensor element, a gap may be provided at only one corner point.

That is, in this gas sensor element, since a gap is provided at only one corner point, a single gap is not provided across multiple corner points. Therefore, it is possible to prevent the gap on the outer surface of the front end portion of the element body from occupying an excessively large area, thereby ensuring a sufficiently large contact area between the element body and the protective layer.

Thus, according to this gas sensor element, since a large contact area between the element body and the protective layer can be ensured, the separation of the protective layer from the element body can be further suppressed.

In the above-mentioned gas sensor element, a plurality of (for example, at least two) gaps may be provided.

If a plurality of gaps are provided, it is possible to prevent condensed water from reaching the plurality of corner points of the element body, thereby suppressing breakage of the element body that might otherwise result from a temperature shock resulting from water adhesion.

In particular, in the gas sensor element described above, the gap may be provided at each of at least two diagonally arranged corner points of the four corner points of the front end of the element body.

In the above-mentioned gas sensor element, the maximum width of the gap measured from the corner point in the width direction of the element body may be smaller than half the width of the element body.

If the maximum width of the gap is less than half the width of the element body, it is possible to prevent the gap from occupying an excessively large area on the outer surface of the front portion of the element body, thereby ensuring a sufficiently large contact area on the outer surface of the element body where the element body is in contact with the protective layer.

In the above-mentioned gas sensor element, the maximum thickness of the gap measured from the corner point in the thickness direction of the element body may be smaller than half the thickness of the element body.

If the maximum thickness of the gap is less than half the thickness of the element body, it is possible to prevent the gap from occupying an excessively large area on the outer surface of the front portion of the element body, thereby ensuring a sufficiently large contact area on the outer surface of the element body where the element body is in contact with the protective layer.

Next, according to an embodiment, the present invention provides a gas sensor comprising a gas sensor element according to any one of the above-mentioned gas sensor elements, which is capable of detecting a specific gas contained in a gas to be measured.

The gas sensor includes any of the above-mentioned gas sensor elements in which breakage of the corner points of the front end of the element body, which might otherwise result from a temperature shock due to adhesion of water, can be suppressed. In addition, the protective layer of the gas sensor element can have a smaller thickness than a conventional protective layer formed by an immersion process and therefore has a smaller heat capacity than the conventional protective layer.

Therefore, the gas sensor of the present invention can be designed to have the gas sensor element whose protective layer has a lower heat capacity than a conventional protective layer.

Next, according to an embodiment, the present invention provides a method for manufacturing a gas sensor element comprising a substantially rectangular parallelepiped element body extending in a longitudinal direction and having in or at its front end portion a detection portion for detecting a specific gas contained in a gas to be measured, and a porous protective layer provided on a front end surface and side surfaces of the element body in such a manner as to cover at least the detection portion.

The gas sensor element manufactured by this manufacturing method has at least one gap formed to separate the element body and the protective layer, the gap being provided only in a front region located in front of a location on the side surfaces of the element body where the protective layer has its greatest thickness. The gap is thus provided at at least one corner point of four corner points of a front end of the element body. that the gap extends over three surfaces of the element body defining the corner point. The protective layer is in contact with at least a portion of the front end surface, and is in contact with at least a portion of the four side surfaces in the front end region.

The method of manufacturing the gas sensor element includes a solvent arranging step of arranging a volatile solvent on an outer surface of the element body in a region where the gap is to be formed, a protective layer forming step of forming, on the element body where the volatile solvent remains, a green protective layer to become the protective layer by heat treatment in such a manner as to cover at least the detection portion, and a heat treatment step of heat treating the element body where the green protective layer is formed, thereby forming the protective layer, wherein the volatile solvent is volatilized during a period from the start of the solvent arranging step to the end of the heat treatment step, thereby forming the gap.

That is, the gap can be formed as a result of volatilization of the volatile solvent between the element body and the protective layer through a series of the following steps: the volatile solvent is arranged in a region where the gap is to be formed on the outer surface of the element body; the green protective layer is formed on the element body where the volatile solvent remains; and heat treatment is performed on the element body where the green protective layer is formed, thereby forming the protective layer.

By the method for manufacturing the gas sensor element using the volatile solvent and comprising the solvent disposing step, the protective layer forming step, and the heat treatment step, the gas sensor element having the gap between the element body and the protective layer can be easily manufactured.

Effect of the invention

The present invention can provide a gas sensor element whose protective layer has a lower heat capacity than a conventional protective layer formed by an immersion process.

Furthermore, the gas sensor of the present invention can be designed to have the gas sensor element whose protective layer has a lower heat capacity than a conventional protective layer.

Furthermore, the method for manufacturing the gas sensor element of the present invention can manufacture the gas sensor element having the gap between the element body and the protective layer and whose protective layer has a lower heat capacity than a conventional protective layer.

Short description of the drawings

• 1 illustrates a sectional view of an air-fuel ratio sensor according to an embodiment of the present invention along an axial direction.
• 2 illustrates a perspective view showing a gas sensor element.
• 3 illustrates an exploded perspective view showing the gas sensor element.
• 4 illustrates a sectional view of the gas sensor element along the line AA in 2 .
• 5 illustrates a sectional view of the gas sensor element along the line BB in 4 .
• 6 illustrates an explanatory view regarding a method for manufacturing green bodies of the gas sensor elements.
• 7 illustrates an explanatory view showing the gas sensor element in the middle of manufacturing.
• 8th illustrates a view showing steps for forming a green protective layer.
• 9 illustrates a sectional view showing a gas sensor element with a non-beveled element body.

Ways to carry out the invention

Next, embodiments of the present invention will be described with reference to the drawings.

The following embodiments will be described with reference to a full-range air-fuel ratio sensor, which may hereinafter be referred to merely as the air-fuel ratio sensor, which is a type of oxygen sensor among gas sensors. In particular, this description will refer to an air-fuel ratio sensor for use in feedback control of the air-fuel ratio in an internal combustion engine, which is mounted on an exhaust pipe of the internal combustion engine and employs a gas sensor element (detecting element) to measure a specific gas (oxygen) contained in a gas to be measured, particularly exhaust gas.

1. First embodiment

1-1 The overall structure

The overall structure of an air-fuel ratio sensor using a gas sensor element of the present invention will be described with reference to 1 to be discribed. 1 is a sectional view showing the internal structure of the air-fuel ratio sensor.

As in 1 As shown, an air-fuel ratio sensor 1 of the present embodiment comprises a tubular metal casing 5 on the outer surface of which a threaded portion 3 is formed, which is suitable for its fixation to an exhaust pipe; a plate-like gas sensor element 7 extending in an axial direction (a longitudinal direction of the air-fuel ratio sensor 1, a vertical direction in 1 ); a tubular ceramic sleeve 9 arranged in such a manner as to radially surround the gas sensor element 7; an insulating contact member 13 (a separator 13) having an insertion hole extending therethrough in the axial direction and arranged such that the inner wall surface of the insertion hole 11 surrounds a rear end portion of the gas sensor element 7; and five connection terminals 15 ( 2 shows only two of them), which are arranged between the gas sensor element 7 and the separator 13.

As will be described later in detail, the gas sensor element 7 includes a rectangular parallelepiped element body 70 extending in the longitudinal direction and a porous protective layer 17 covering a front end portion of the element body 70. The element body 70 has a detection portion 90 provided in its front end region and adapted to detect a specific gas contained in a gas to be measured. In addition, the gas sensor element 7 has electrode pads 25, 27, 29, 31, and 33 provided on the outer surface of its rear end portion (an upper end portion in 1 , a longitudinally rear end portion), in particular on a first main surface 21 and a second main surface 23 of the rear end portion (see 2 and 3 for details).

The connection terminals 15 are electrically connected to the electrode pads 25, 27, 29, 31, and 33 of the gas sensor element 7, respectively, and are also electrically connected to corresponding lead wires 35 extending from the outside into the sensor, thereby forming electric current paths through which electric current flows between an external device connected to the lead wires 35 and the electrode pads 25, 27, 29, 31, and 33.

The metal case 5 has a substantially tubular shape and is designed to have a through hole 37 extending therethrough in the axial direction and a step 39 projecting radially inward from the wall surface of the through hole 37. The metal case 5 holds the gas sensor element 7 inserted through the through hole 37 in such a manner that the detection portion 90 is disposed in front of the front end of the through hole 37 while the electrode pads 25, 27, 29, 31, and 33 are disposed behind the rear end of the through hole 37.

In addition, in the through hole 37 of the metal case 5, an annular ceramic holder 41, a talc ring 43, a talc ring 45 and the ceramic sleeve 9 are stacked in this order toward the rear in such a manner as to radially surround the gas sensor element 7.

A crimp seal 49 is disposed between the ceramic sleeve 9 and a rear end portion 47 of the metal housing 5, while a metal holder 51 for holding the talc ring 43 and the ceramic holder 41 is disposed between the ceramic holder 41 and the shoulder 39 of the metal housing 5. The rear end portion 47 of the metal housing 5 is crimped in such a manner that the ceramic sleeve 9 is pressed forward by the crimp seal 49.

In addition, a protector 55 made of metal (e.g., stainless steel) and having a double structure is attached, for example, by welding, to the outer periphery of a front end portion 53 of the metal case 5 and covers a projecting portion of the gas sensor element 7.

On the other hand, an outer tube 57 is fixed to the outer periphery of a rear portion of the metal case 5. A bushing 61 in which lead wire insertion holes 59 are formed is arranged in a rear opening of the outer tube 57, and the five lead wires 35 ( 1 shows three of them) are inserted through the respective lead wire insertion holes 59 and electrically connected to the electrode pads 25, 27, 29, 31, and 33, respectively.

The separator 13 has a collar portion 63 formed on its outer periphery, and the collar portion 63 is fixed to the outer tube 57 by a holding member 65.

1-2. The structure of the gas sensor element

Next, the structure of the gas sensor element 7, which is an essential element of the present embodiment, will be described with reference to 2 to 5 be described in detail.

2 is a perspective view showing the appearance of the gas sensor element 7.

As in 2 shown, the gas sensor element 7 is an elongated plate element extending in the longitudinal direction (the direction of the Y-axis). In 2 the longitudinal direction corresponds to the axial direction of the gas sensor. In addition, 2 a Z-axis direction is a thickness direction perpendicular to the longitudinal direction, and an X-axis direction is a width direction perpendicular to the longitudinal direction and the thickness direction.

The gas sensor element 7 includes a rectangular parallelepiped element body 70 extending in the longitudinal direction and the porous protective layer 17 having a front end portion (a lower end portion in 2 ) of the element body 70. The element body 70 is configured such that a plate-like member 71 extending in the longitudinal direction and a plate-like heating element 73 extending in the longitudinal direction are laminated together. The element body 70 has the detection portion 90 provided at or in its front end region and capable of detecting a specific gas contained in the gas to be measured. The protective layer 17 is provided on a front end surface 127 and side surfaces (the first main surface 21, the second main surface 23, a first side surface 111, and a second side surface 113) of the element body 70 in such a manner as to cover at least the detection portion 90.

3 is an exploded perspective view showing the gas sensor element 7. 3 omits the illustration of the protective layer 17 as well as a first longitudinal bevel 121, a second longitudinal bevel 122, a third longitudinal bevel 123, and a fourth longitudinal bevel 124, which will be described later.

As in 3 shown in an exploded state, the element body 70 of the gas sensor element 7 comprises the plate-like element 71 which is laminated in a lamination direction on one side (the upper side in 3 ) and extending in the longitudinal direction, and the plate-like heating element 73 which is arranged on an opposite side (the back side) to the element 71 and extending in the longitudinal direction.

The element 71 includes an oxygen concentration cell 81 configured such that porous electrodes 77 and 79 are formed on respective opposite sides of a solid electrolyte body 75; an oxygen pump cell 89 configured such that porous electrodes 85 and 87 are formed on respective opposite sides of a solid electrolyte body 83; and an insulating spacer 93 laminated between the two cells 81 and 89 and in which a hollow gas measuring chamber 91 is formed. The solid electrolyte bodies 75 and 83 are formed of zirconia containing yttria as a stabilizer in a solid solution, and the porous electrodes 77, 79, 85, and 87 are formed mainly of Pt.

The insulating spacer 93 in which the gas measuring chamber 91 is formed is mainly made of alumina. The porous electrode 77 of the oxygen concentration cell 81 and the porous electrode 87 of the oxygen pump cell 89 are arranged in such a manner as to be exposed to the hollow gas measuring chamber 91.

The member 71 has two gas inlets 94 formed in respective sides thereof (sides of the insulating spacer 93). The gas inlets 94 serve as inlets for exhaust gas (the gas to be measured) and communicate with the gas measuring chamber 91. In respective paths extending from the two gas inlets 94 to the gas measuring chamber 91, diffusion control portions 95 are formed. The diffusion control portions 95 are porous bodies formed of, for example, alumina, and control the diffusion of the gas to be measured flowing into the gas measuring chamber 91. The diffusion control portions 95 are partially exposed from the gas inlets 94.

That is, in the gas sensor element 7, the gas inlets 94 are formed in the outermost surfaces of the element body 70 and face in two different directions, and the diffusion control portions 95 are exposed in the two different directions.

In addition, an insulating substrate 97 formed mainly of aluminum oxide is deposited on the side of the first main surface 21 (the upper side in 3 ) of the element 71. The insulating substrate 97 has a vent portion 99 embedded therein, and the vent portion 99 is a porous body similarly to the case of the diffusion control portions 95. The vent portion 99 allows the porous electrode 85 of the oxygen pump cell 89 to be exposed to the gas to be measured.

The gas measuring chamber 91 is located in a front end area (a left end area in 3 ) of the element body 70. Referring to the longitudinal direction of the element 71, a region in which the gas measuring chamber 91 is formed and a region located in front of the gas measuring chamber 91 constitute the detection section 90 for detecting oxygen.

On the other hand, the heating element 73 is formed such that a heat generating resistor pattern 105 mainly made of Pt is interposed between insulating substrates 101 and 103 mainly made of alumina.

The gas sensor element 7 has three electrode pads 25, 27, and 29 formed at a rear end portion (a right end portion in 3 ) of the first main surface 21, and the two electrode terminal surfaces 31 and 33 formed at a rear end portion of the second main surface 23.

As in 3 shown, the electrode pad 29 formed on the first main surface 21 (the right electrode pad in 2 ) is electrically connected to the porous electrode 77 of the oxygen concentration cell 81 exposed to the gas measuring chamber 91 through a through hole 161 provided in the insulating substrate 97, a through hole 165 provided in the solid electrolyte body 83, and a through hole 171 provided in the insulating spacer 93. The electrode pad 29 is also electrically connected to the porous electrode 87 of the oxygen pump cell 89 exposed to the gas measuring chamber 91 through the through hole 161 provided in the insulating substrate 97 and the through hole 165 provided in the solid electrolyte body 83. Thus, the porous electrode 77 and the porous electrode 87 are electrically connected to each other and therefore have the same electric potential.

In addition, as in 3 shown another electrode pad 27 (the middle electrode pad in 2 ) through a through hole 162 provided in the insulating substrate 97, a through hole 166 provided in the solid electrolyte body 83, a through through hole 172 provided in the insulating spacer 93 and through hole 176 provided in the solid electrolyte body 75, electrically connected to the other porous electrode 79 of the oxygen concentration cell 81. In addition, as shown in 3 shown a further electrode connection surface 25 (the left electrode connection surface in 2 ) is electrically connected to the other porous electrode 85 of the oxygen pump cell 89 through a through hole 163 provided in the insulating substrate 97.

In addition, as in 3 As shown, the electrode pads 31 and 33 are connected to respective opposite ends of the heat-generating resistor pattern 105 through through holes 181 and 182 formed in the insulating substrate 103, respectively.

Since the gas sensor element 7 designed in this way, with reference again to 2 is an elongated, substantially rectangular parallelepiped plate element, it has four edges (longitudinal ridge lines) H1, H2, H3, and H4 extending in its longitudinal direction (Y-axis direction in 2 ).

More specifically, the gas sensor element 7 has four outer walls extending in its longitudinal direction; that is, the first main surface 21 and the second main surface 23, and the first side surface 111 and the second side surface 113 in contact with the first main surface 21 and the second main surface 23. In addition, the gas sensor element 7 has the first edge H1 which is a ridge line between the first main surface 21 and the first side surface 111; the second edge H2 which is a ridge line between the first main surface 21 and the second side surface 113; the third edge H3 which is a ridge line between the second main surface 23 and the second side surface 113; and the fourth edge H4 which is a ridge line between the second main surface 23 and the first side surface 111.

The first edge H1, the second edge H2, the third edge H3, and the fourth edge H4 are bevelled by 0.2 mm, thereby forming a first longitudinal edge bevel 121, a second longitudinal edge bevel 122, a third longitudinal edge bevel 123 (see 4 ) and a fourth longitudinal edge bevel 124 (see 4 ). In 2 The third longitudinal edge bevel 123 and the fourth longitudinal edge bevel 124 are not shown with reference symbols since they are not visible.

The gas sensor element 7 is also provided with four ridge lines of its rear end face 129 (the upper end face in 2 ) to form bevels 131 at the rear end while leaving the central rear end surface 129 (perpendicular to the longitudinal direction).

Next, gaps 18 formed between the element body 70 and the protective layer 17 of the gas sensor element 7 will be described.

4 is a sectional view of the gas sensor element 7 along the line AA in 2 . 5 is a sectional view of the gas sensor element 7 along the line BB in 4 .

The protective layer 17 is formed of porous alumina and covers at least the detection portion 90 of the element body 70. The gaps 18 are formed between the protective layer 17 and the element body 70 so as to separate the protective layer 17 and the element body 70.

The number of the gaps 18 is at least four. The gaps 18 are formed to correspond to four corner points 74 of the front end surface 127 of the element body 70. Each gap 18 is formed at a single corner point 74 so as to extend over three surfaces of the element body 70 defining the single corner point 74. For example, the gap 18 of the four gaps 18 shown in 4 shown, the upper left gap 18 is formed at a single corner point 74 such that the gap 18 extends over the first main surface 21, the front end surface 127 and the first side surface 111 of the element body 70.

Since the element body 70 of the present embodiment has the first longitudinal edge bevel 121, the second longitudinal edge bevel 122, the third longitudinal edge bevel 123, and the fourth longitudinal edge bevel 124, each of the four corner points 74 takes the shape of a ridge line defined by the front end surface 127 and each of the first longitudinal edge bevel 121, the second longitudinal edge bevel 122, the third longitudinal edge bevel 123, and the fourth longitudinal edge bevel 124.

In addition, the gaps 18 as in 5 shown on the side surfaces (the first main surface 21, the second main surface 23, the first side surface 111, and the second side surface 113) of the element body 70 only in a front region E1 located in front of a location 19. The location 19 on the element body 70 is a location on the side surfaces (the first main surface 21, the second main surface 23, the first side surface 111, and the second side surface 113) of the element body 70 at which the protective layer 17 has its greatest thickness Dmax.

In the gas sensor element 7, the gaps 18 are provided at those respective corner points 74 of the front end surface 127 of the element body 70 where the thickness of the protective layer 17 tends to decrease, extending over three surfaces of the element body 70 defining each of the corner points 74. Thus, the element body 70 and the protective layer 17 are separated. Therefore, breakage of the corner points 74 of the front end of the element body 70, which might otherwise result from a temperature shock resulting from adhesion of water, can be suppressed, although the thicknesses L1 to L4 of the protective layer 17 near the corner points 74 are small. This is because the gaps 18 provided at the corner points 74 cause condensed water adhering to the protective layer 17 to penetrate into the protective layer 17 bypassing the gaps 18, thereby preventing the condensed water from reaching the corner points 74.

In addition, the gaps 18 are formed only in the front region E1 located in front of the position 19 on the four side surfaces (the first main surface 21, the second main surface 23, the first side surface 111, and the second side surface 113) of the element body 70 where the protective layer 17 has the largest thickness Dmax. Therefore, the gaps 18 can prevent the condensed water from reaching the corner points 74. In addition, it is possible to suppress deterioration of the adhesion between the protective layer 17 and the element body 70 in a rear region E2 located behind the position 19.

In addition, the protective layer 17 is in contact with at least a portion of the front end surface 127, and is in contact with at least a portion of the four side surfaces (the first main surface 21, the second main surface 23, the first side surface 111, and the second side surface 113) in the front region E1. Thereby, despite the fact that the gaps 18 formed to separate the element body 70 and the protective layer 17 are provided in the front region E1, deterioration of the adhesion between the protective layer 17 and the element body 70 in the front region E1 can be suppressed. In particular, since the protective layer 17 is in contact with at least a portion of the four side surfaces of the element body 70 in the front region E1, it is possible to further suppress deterioration of the adhesion between the protective layer 17 and the element body 70 in the front region E1.

In addition, the element body 70 is in contact with the protective layer 17 in the front region E1 through a portion of the front end surface 127 and portions of the side surfaces (the first main surface 21, the second main surface 23, the first side surface 111, and the second side surface 113) in the front region E1, the total area of the portion of the front end surface 127 and the portions of the side surfaces being half of the total area of the front end surface 127 and the side surfaces. Since a sufficient contact area between the element body 70 and the protective layer 17 can be ensured in the front end region E1, it becomes even less likely that the protective layer 17 will separate from the element body 70.

As in 4 shown, the maximum width W1 of each gap 18 measured from the corresponding corner point 74 in the width direction of the element body 70 is smaller than half the width W2 of the element body 70. Particularly, when the corner point 74 is not a point but a side of a tapered portion, the distance between a corner point existing before the formation of the tapered portion and an end portion of the gap 18 closest to the center of the element body 70 in the width direction thereof is used as the maximum width W1. In addition, when as in 4 shown several gaps 18 are provided, preferably that the maximum widths W1 of all gaps 18 are smaller than half the width W2 of the element body 70.

When the maximum width W1 of each gap 18 is smaller than half the width W2 of the element body 70, the gap 18 can be prevented from occupying an excessively large area on the outer surface of the front portion E1 of the element body 70, thereby ensuring a sufficiently large contact area between the element body 70 and the protective layer 17.

In addition, as in 4 shown, the maximum thickness T1 of each gap 18 measured from the corresponding corner point 74 in the thickness direction of the element body 70 is smaller than half the thickness T2 of the element body 70. Particularly, when the corner point 74 is not a point but a side of a tapered portion, the distance between a corner point existing before the formation of the tapered portion and an end portion of the gap 18 closest to the center of the element body 70 in the thickness direction thereof is used as the maximum thickness T1. In addition, when as in 4 shown several gaps 18 are provided, preferably that the maximum thicknesses T1 of all gaps 18 are less than half the thickness T2 of the element body 70.

When the maximum thickness T1 of each gap 18 is smaller than half the thickness T2 of the element body 70, the gap 18 can be prevented from occupying an excessively large area on the outer surface of the front portion E1 of the element body 70, thereby ensuring a sufficiently large contact area between the element body 70 and the protective layer 17.

1-3. The process for manufacturing the gas sensor

With reference to 6 and 7 A method of manufacturing the air-fuel ratio sensor 1 of the present embodiment will be described.

6 is an explanatory view regarding a method of manufacturing a green body 141 of the gas sensor elements. 7 is an explanatory view showing the gas sensor element in the middle of manufacturing.

In manufacturing the gas sensor element 7, a non-compressed laminate is first prepared by laminating various well-known materials used to form the gas sensor element 7; specifically, among others, green solid electrolyte sheets used to form the solid electrolyte bodies 75 and 83 of the element 71 and green insulating sheets used to form the insulating substrates 97, 101, and 103 of the heater element 73 and the element 71. In the non-compressed laminate, among others, pre-formed green electrode pads to become the electrode pads 25, 27, 29, 31, and 33 are present.

Among these materials, for example, the green solid electrode plate is formed in the following way. First, alumina powder, butyral resin, etc. are added to a ceramic powder mainly containing zirconia. A mixed solvent (toluene and methyl ethyl ketone) is mixed into the resulting mixture to form a slurry. The slurry is made into a plate by a doctor blade process, and the mixed solvent is volatilized to form the green solid electrolytic plate.

Further, the green insulation board is formed in the following manner. First, butyral resin and dibutyl phthalate are added to a ceramic powder mainly containing alumina. A mixed solvent (toluene and methyl ethyl ketone) is mixed into the resulting mixture to form a slurry. The slurry is made into a board by a doctor blade process, and the mixed solvent is volatilized to give the green insulation board.

In addition, green diffusion control portions are formed in the following manner. First, 100 mass% alumina powder and a plasticizer are wet-mixed to form a slurry in which the alumina powder and the plasticizer are dispersed. The plasticizer contains butyral resin and DBP. The slurry is applied to areas where the diffusion control portions 95 and the vent portion 99 are to be formed by firing, thereby forming the green diffusion control portions.

Then the uncompressed laminate is compressed under a pressure of 1 MPa, which creates the 6 The method for producing the non-compressed laminate is similar to a well-known method for producing a gas sensor element; therefore, a detailed description of the method is omitted.

The green body 141 produced by the application of pressure is cut into several (e.g., ten) laminates, each of which has a certain size substantially identical to that of the element 71 and the heating element 73 of the gas sensor element 7.

The green laminate is then debound and subjected to a one-hour normal firing at 1,500 °C, which produces a 7 shown fired laminate 143.

Next, the fired laminate 143 is beveled at its four longitudinal edges (the first edge H1, the second edge H2, the third edge H3, and the fourth edge H4), thereby forming the first longitudinal edge bevel 121, the second longitudinal edge bevel 122, the third longitudinal edge bevel 123, and the fourth longitudinal edge bevel 124 (see 2 and 4 ). Specifically, the four longitudinal edges (the first edge H1, the second edge H2, the third edge H3, and the fourth edge H4) are subjected to a grindstone for well-known beveling. This gives the element body 70.

After the element body 7 is produced as mentioned above, a green protective layer is formed around a front end portion of the element body 70. The green protective layer is fired into the protective layer 17 with the gaps (see 2 , 4 and 5 ).

8th is an explanatory view showing steps of forming a green protective layer 117 on the element body 70 and producing the protective layer 17 by performing heat treatment on the green protective layer 117.

In a stage before the formation of the green protective layer 117, no volatile solvent 118 and no green protective layer 117 are present on the element body 70.

First, in the first step, the volatile solvent 118 (e.g., ethanol, propylene glycol, or butyl carbitol) is applied to the four corner points 74 of the front end surface 127 of the element body 70. At this time, the volatile solvent 118 is applied to each corner point 74 in such a manner that the volatile solvent 118 extends along three surfaces of the element body 70 defining the corner point 74. That is, in the first step, the volatile solvent 118 is applied to areas that are to become the gaps 18 after firing.

In the next step, while the volatile solvent 118 is left, a front end portion of the element body is immersed in a protective layer slurry in a slurry tank, whereby a portion of a green protective layer 117 is formed on the element body 70 in such a manner as to cover at least the detection portion 90.

In the subsequent third step, the front end portion of the element body 70 is again immersed in the protective layer slurry in the slurry tank, thereby forming another portion of the green protective layer 117. Then, the immersion is performed for a certain number of times, thereby completing the formation of the green protective layer 117 having a certain thickness.

In the subsequent fourth step, the green protective layer 117 is subjected to heat treatment. Specifically, the element body 70 with the green protective layer 117 formed thereon is subjected to heat treatment at a temperature of 1,000°C for three hours, resulting in the gas sensor element 7 on which the protective layer 17 with the gaps 18 is formed. The green protective layer 117 is fired to thereby become the protective layer 17, and the portions to which the volatile solvent 118 has been applied become the gaps 18.

During the period from the application of the volatile solvent 118 in the first step to the heat treatment of the green protective layer 117 in the fourth step, the volatile solvent 118 gradually evaporates, thereby forming the gaps 18 between the element body 70 and the protective layer 17.

The gas sensor element 7 thus formed is mounted on the metal housing 5 in a partial assembly step.

Specifically, in this partial assembly step, the gas sensor element 7 manufactured by the above method is inserted into the metal holder 51; further, the gas sensor element 7 is fixed in place by the ceramic holder 41 and the talc ring 43, which forms a partial assembly. By fixing the partial assembly to the metal case 5 and inserting a rear end in the axial direction section of the gas sensor element 7 through the talc ring 45 and the ceramic sleeve, these elements are then inserted into the metal housing 5.

Then, the rear end portion 47 of the metal housing 5 is crimped to the ceramic sleeve 9, resulting in a lower subassembly. The protective device 55 was previously attached to the lower subassembly.

Next, the outer tube 57, the separator 13, the bushing 61, etc. are assembled to form an upper subassembly. Then, the lower subassembly and the upper subassembly are joined together to form the air-fuel ratio sensor 1.

1-4. Comparison test

In order to check the water adhesion resistance of the gas sensor element of the present invention, a water adhesion test was conducted. The test results will be described below.

In this test, a certain amount of water was caused to adhere to the protective layer 17 of the gas sensor element 7 to determine whether the element body 70 was broken or not. A sensor signal Ip output from the gas sensor element 7 was monitored. If the change in the monitored sensor signal Ip relative to the sensor signal Ip before the water adhered was equal to or greater than 1%, the element body 70 was considered to be broken. If the change in the monitored sensor signal Ip relative to the sensor signal Ip before the water adhered was less than 1%, the element body 70 was considered to be not broken.

In this test, a gas sensor element having no gaps between the protective layer and the element body was prepared as a comparative example and subjected to the water adhesion test. The water adhesion test was conducted on two test pieces (Examples 1 and 2) of the present invention which differed in the thickness of the protective layer at corners of the front end of the gas sensor element and on two test pieces (Comparative Examples 1 and 2) which differed in the thickness of the protective layer at corners of the front end of the gas sensor element.

In the experiment, the amount of water adhering to the specimen was increased in five steps (1 µl, 2 µl, 5 µl, 7 µl, and 10 µl). If a particular specimen fractured at a certain amount of water adhering to the specimen, the experiment was stopped at that level of water adhering to the specimen.

Table 1 shows the test results. In Table 1, the rating “good” in the column “Result of water adhesion test” indicates that the element body did not break, and “poor” indicates that the element body did break. Table 1 Test piece Space available Result of the water adhesion test Thickness of the protective layer (µm) 1 µl 2 µl 5 µl 7 µl 10 µl example 1 Yes good good good good good 344.4 Example 2 Yes good good good good good 354.5 Comparison example 1 no good bad - - - 330.4 Comparison example 2 no good good bad - - 351.2

According to the test results, no breakage of the element body occurred in Examples 1 and 2 at any test level of the amount of adhering water (1 µm to 10 µm). Comparative Example 1 experienced breakage of the element body at an amount of adhering water of 2 µm, and Comparative Example 2 experienced breakage of the element body at an amount of water adhesion of 5 µm.

Thus, the gas sensor element 7 of the present invention is less likely to cause breakage of the element body, which might otherwise result from a thermal shock, compared with a gas sensor element having no gaps, and therefore has excellent resistance to water adhesion.

1-5. Effects

As described above, in the air-fuel ratio sensor 1 of the present embodiment, the gas sensor element 7 has the gaps 18 between the protective layer 17 and the element body 70.

The gas sensor element 7 has at least four gaps 18 corresponding to four corner points 74 of the front end of the element body 70. Each gap 18 is provided at the corresponding corner point 74 so that the gap 18 extends over three surfaces of the element body 70 defining the corner point 74. In addition, the gaps 18 are formed only in the front region E1 located in front of the position 19 (where the protective layer 17 has the largest thickness Dmax) on the side surfaces (the first main surface 21, the second main surface 23, the first side surface 111, and the second side surface 113) of the element body 70.

In the gas sensor element 7, the gaps 18 for separating the element body 70 and the protective layer 17 are formed at the corner points 74 of the front end of the element body 70 where the thickness of the protective layer 17 is likely to become small, such that each gap 18 extends over three surfaces of the element body 70 defining the corresponding corner point 74. Therefore, despite the fact that the thicknesses L1 to L4 of the protective layer 17 are small in the vicinity of the corner points 74, breakage of the corner points of the front end of the element body, which might otherwise result from a temperature shock resulting from adhesion of water, can be suppressed.

That is, compared with a conventional protective layer formed by an immersion process, the thickness, and thereby the heat capacity, of the protective layer 17 of the gas sensor element 7 can be reduced.

In addition, the gaps 18 are formed only in the front region E1 located in front of the position 19 on the side surfaces of the element body 70 where the protective layer 17 has the largest thickness Dmax. Thanks to this feature, it is possible to prevent condensed water from reaching the corner points 74 through the gaps 18, and it is also possible to suppress deterioration of the adhesion between the protective layer 17 and the element body 70 in the rear region E2 located behind the position 19.

In addition, the protective layer 17 is in contact with at least a portion of the front end surface 127 of the element body 70, and is in contact with at least a portion of the four side surfaces (the first main surface 21, the second main surface 23, the first side surface 111, and the second side surface 113) in the front region E1. Thus, even though the gaps 18 formed in such a manner as to separate the element body 70 and the protective layer 17 are provided in the front region E1, deterioration of the adhesion between the protective layer 17 and the element body 70 can be suppressed even in the front region E1.

1-6. Correspondence between the claims and the present embodiment

Hereinafter, the correspondence in wording between the claims and the present embodiment will be described.

The first main surface 21, the second main surface 23, the first side surface 111, and the second side surface 113 correspond to the side surfaces of the main body. The air-fuel ratio sensor 1 corresponds to the gas sensor.

The first step corresponds to the solvent arrangement step; the second and third steps correspond to the protective layer formation step; and the fourth step corresponds to the heat treatment step.

2. Other embodiment

Although the present invention has been described with reference to the above embodiment, the present invention is not limited thereto, but may be embodied in various other forms without departing from the gist of the invention.

For example, the element body of the gas sensor element is not limited to one in which the first edge H1, the second edge H2, the third edge H3, and the fourth edge H4 are chamfered. The element body may be designed such that none of the first edge H1, the second edge H2, the third edge H3, and the fourth edge H4 is chamfered.

9 is a sectional view showing a second gas sensor element 207 having a non-beveled second element body 270 along the front end surface 127 of the second element body 270. In the second gas sensor element 207 thus designed, each of the corner points 74 takes the form of a single point rather than a ridge line. Therefore, the maximum width W1 and the maximum thickness T1 of each gap 18 are as shown in 9 shown determined.

The gaps are not necessarily provided at all four corner points of the front end face of the element body as in the case of the first embodiment. For example, the gaps may be provided at three or less of the four corner points. In certain uses of the gas sensor element, the provision of a single gap may be able to suppress a temperature shock resulting from adhesion of water. In such a case, the provision of a single gap may be employed. In the case of providing two or more gaps at the respective corner points, the gaps may be provided at at least two diagonally located corner points, respectively.

A single gap is not necessarily provided at only a single vertex, but may be provided to extend between and across two vertices.

In the case of providing a large number of gaps or providing large gaps, it is preferable to provide a sufficient contact area between the protective layer and the element body. For example, by applying contact between the protective layer and the element body to at least half of the area of the front end surface and the side surfaces in the front region E1 of the element body, a gas sensor element in which separation of the protective layer from the element body is unlikely to occur can be realized.

Description of reference symbols

1: air-fuel ratio sensor; 5: metal case; 7: gas sensor element; 9: ceramic sleeve; 11: insertion hole; 17: protective layer; 18: gap; 19: location; 21: first main surface; 23: second main surface; 70: element body; 74: corner point; 81: oxygen concentration cell; 89: oxygen pump cell; 90: detection section; 91: gas measuring chamber; 94: gas inlet; 95: diffusion control sections; 105: heat generating resistance pattern; 111: first side surface; 113: second side surface; 117: green protective layer; 118: volatile solvent; 127: front end surface; 143: fired laminate; 207: second gas sensor element; 270: second element body

Claims (10)

A gas sensor element (7, 207) comprising: a substantially rectangular parallelepiped element body (70, 270) extending in a longitudinal direction and having in or at its front end portion a detection portion (90) for detecting a specific gas contained in a gas to be measured; and a porous protective layer (17) provided on a front end surface (127) and side surfaces (21, 23, 111, 113) of the element body (70, 270) in such a manner as to cover at least the detection portion (90), wherein the gas sensor element (7, 207) has at least one gap (18) formed to separate the element body (70, 270) and the protective layer (17), the gap (18) being provided only in a front region (E1) located in front of a location (19) on the side surfaces of the element body (70, 270) where the protective layer (17) has its greatest thickness; wherein the gap (18) is formed at at least one corner point of four corner points (74) of a front end of the element body (70, 270) such that the gap extends over three surfaces of the element body (70, 270) defining the corner point; and wherein the protective layer (17) is in contact with at least a portion of the front end surface (127) and is in contact with at least a portion of the four side surfaces (21, 23, 111, 113) in the front region. Gas sensor element according to Claim 1 wherein the protective layer (17) is in contact with at least a portion of each of the four side surfaces (21, 23, 111, 113) in the front region. Gas sensor element according to Claim 1 or 2 , wherein the element body (70) is in contact with the protective layer (17) via a portion of the front end surface (127) and portions of the side surfaces (21, 23, 111, 113) in the front region, wherein the total surface area of the portion of the front end surface (127) and the portions of the side surfaces (21, 23, 111, 113) is half the total surface area of the front end surface (127) and the side surfaces (21, 23, 111, 113). Gas sensor element according to one of the Claims 1 until 3 , wherein a gap (18) is provided at only one corner point. Gas sensor element according to one of the Claims 1 until 3 , wherein a plurality of intermediate spaces (18) are provided. Gas sensor element according to Claim 5 wherein the gap (18) is provided at each of at least two diagonally arranged corner points of the four corner points (74) of the front end of the element body (70, 270). Gas sensor element according to one of the Claims 1 until 6 , wherein a maximum width (W1) of the gap (18) measured from the corner point in a width direction of the element body (70, 270) is smaller than half a width (W2) of the element body (70, 270). Gas sensor element according to one of the Claims 1 until 7 , wherein a maximum thickness (T1) of the gap (18) measured from the corner point in a thickness direction of the element body (70, 270) is less than half a thickness (T2) of the element body (70, 270). Gas sensor (1) comprising a gas sensor element (7, 207) adapted to measure a specific gas contained in a gas to be measured, according to one of the Claims 1 until 8th . A method of manufacturing a gas sensor element (7, 207) comprising a substantially rectangular parallelepiped element body (70, 270) extending in a longitudinal direction and having in its front end region a detection portion (90) for detecting a specific gas contained in a gas to be measured, and a porous protective layer (17) provided on a front end surface (127) and side surfaces (21, 23, 111, 113) of the element body (70, 270) in such a manner as to cover at least the detection portion, the gas sensor element (7, 207) having at least one gap (18) formed to separate the element body (70, 270) and the protective layer (17), the gap (18) being formed only in a front region (E1) located in front of a location (19) on the side surfaces of the element body (70, 270) at which the protective layer (17) has its greatest thickness; wherein the gap (18) is provided at at least one corner point of four corner points (74) of a front end of the element body (70, 270) such that the gap (18) extends over three surfaces of the element body (70, 270) defining the corner point; and wherein the protective layer (17) is in contact with at least a portion of the front end surface (127), and is in contact with at least a portion of the four side surfaces (21, 23, 111, 113) in the front region, the method comprising: a solvent disposing step (first step) of disposing a volatile solvent on an outer surface of the element body (70, 270) in a region where the gap (18) is to be formed, a protective layer forming step (second and third steps) of forming on the element body (70, 270) where the volatile solvent has remained, a green protective layer to become the protective layer (17) by a heat treatment in such a manner as to cover at least the detection portion (90), and a heat treatment step (fourth step) of heat treating the element body (70, 270) on which the green protective layer is formed, thereby forming the protective layer (17), wherein the volatile solvent is volatilized during a period from the start of the solvent arranging step to the end of the heat treatment step, thereby forming the gap (18).

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